# How feasible is it to use aluminum and liquid oxygen as future propellant sourced from the moon?

I was looking into whether there would be any way to source propellants from the moon itself in support of a future base and space stations in cis-lunar space. I found two documents exploring how aluminum in particular might work in combination with liquid oxygen.

One is by Wickman Spacecraft & Propulsion, who say:

An additional option available with aluminum is to suspend the aluminum powder in gelled LOX to form a monopropellant... As part of our research, we made a small rocket engine fueled by the LOX-aluminum monopropellant... The propellant tank was surrounded by a liquid nitrogen bath to keep the LOX from boiling off. The propellant feed lines ran through a liquid nitrogen bath on their way to the combustion chamber. A piston pushed against the propellant to feed the propellant into the rocket engine chamber. While the thrust was only about a pound, the engine was started and stopped several times without a flashback of the combustion flame front into the propellant tank.

The other is from a document called the Moon Miner's Manifesto, hosted on the website of the Artemis Society International:

Aluminum and oxygen alone will provide a specific impulse somewhat lower than most hydrocarbons. Brower et al. expect a value of 285 seconds... One [engine design] would be to pump aluminum powder as we do fluids. In this case, it will probably be necessary to use a carrier gas along with the powder to keep the aluminum grains from vacuum welding or sticking together from electrostatic forces... Another technique is a hybrid rocket engine using solid aluminum and liquid oxygen. A conceptual design for such an engine was proposed by Brower et al. Their design calls for a hexagonal array of aluminum bars the length of the combustion chamber. Liquid oxygen would be fed down the bars for regenerative cooling before reaching the flame at the bar tips. The engine could use oxygen and aluminum only, or could use tripropellant operation with hydrogen.

The Brower et al. paper was presented at the 26th Joint Propulsion Conference in 1990 and is in the catalogue of the AIAA.

Obviously this needs a great deal of development, but at first blush it seems like it could have advantages over the usual proposal at this point - acquiring water ice from asteroids, or from permanently shaded craters at the moon's poles (if present in suitable quantities and concentrations), and splitting it into hydrogen and oxygen for use in H2/LOX engines.

The lunar highlands are composed largely of anorthite (CaAl2Si2O8), a mineral that is 15% aluminum and 62% oxygen. Regions that are 80% anorthite or more seem to be common. Extraction of oxygen has been discussed in a number of papers, it requires heating regolith above 1100 Celsius so that the mineral oxides dissociate and the oxygen can be collected.

Fine powder feedstock... into an arc discharge to vaporize and ionize... electrostatic acceleration, then electromagnets to mass separate. Power with a solar panel array. No moving parts (except for the powder feed). No vacuum systems needed. And you separate all of the elements at once.

It has been pointed out to me by TildalWave that this would require a great deal of electricity and coolant. It occurred to me that perhaps the coolant could be avoided by putting the whole separator mechanism on rails, on a long shaded track running beside your solar panel field. If you set up the separator to dump the metal and slag products directly onto the ground under the track as it moves along, perhaps it is just a matter of figuring out how to get the flow and the rate at which the separator moves right, and the cooling could happen passively. There would be linear trails of metals and slag left on the ground which could slowly radiate away their heat until they are cool enough to be collected. There are probably reasons that wouldn't work, but I need to be told what they are. The oxygen that off-gasses could be collected from within the open-bottom chamber where the vaporized streams are ejected by the separator.

Could some process along these lines be competitive with asteroid mining? What are the issues and hurdles?

• Using magnesium, phosphorus, or sulfur sourced from the moon in combination with LOX was also explored in the Wickman paper. Btw i want to say i may have trouble responding here over the weekend, but i am intensely interested in this and will be on it on Monday if i can't earlier. Nov 14, 2014 at 18:18
• Wouldn't separating out the aluminum require something along the lines of a robotic auto-smelter (probably at very high temperatures or with caustic chemicals)? The big advantage I see of using water (to electrolyze into its components as fuel and oxidizer) is that water would be easier to extract out since its evaporation temp is so much lower. I just did a search and found that there are huge quantities of Aluminum oxide and anorthosite (I never knew!) in the regolith. I wonder how feasible straight aluminum-oxide electrolysis is... anyway, interesting question! Nov 15, 2014 at 1:45
• I have a paper by C.L. Senior called 'Lunar Oxygen Production by Pyrolysis'. I don't remember where i got it from, i found it in my files. In it he suggested temperatures up to 3000K could be obtained by concentrating sunlight, and used for vapor separation. I haven't got time to go over it more now. I'm certainly loving the thought of this. Nov 15, 2014 at 2:22
• Here on earth alumina is dissolved in molten cryolite. The dissolved alumina is split to aluminum and oxygen via electrolysis. An energy intensive process both in terms of thermal as well as electric watts. Jul 24, 2015 at 14:51
• Does solar pyrolysis entail a transparent, air tight kiln? Abrasive lunar dust would tear the hell out of seals and gaskets. Would rocks explode when being heated to 3000K? I'm picturing the kiln containing a turbulent cloud of superheated gas and dust. A cloud that would sandblast a transparent wall making it opaque. Jul 24, 2015 at 15:00

There are several issues at play here.

1. Are the raw materials needed to be found on the moon?
2. Can the raw materials be realistically harvested on the moon?
3. Can the raw materials be processed into a rocket propellant on the moon?
4. Are rocket engines which use this propellant feasible?
5. Are there more suitable materials for this purpose to be found on the moon?

Let's look at them in turn:

## Are the raw materials needed to be found on the moon?

As per the OP's cited paper the materials exist in sufficient quantity on the moon. This is well confirmed via many sources, as is shown at Wikipedia.

## Can the raw materials be realistically harvested on the moon?

Yes they can, and in fact were "harvested" by Apollo. Much refining would be required, but the raw materials can easily be harvested.

## Can the raw materials be processed into a rocket propellant on the moon?

As per the paper cited above, the rocket propellant is a suspension of aluminum in LOX. Realistically, I do not foresee this suspension being used in actual production however the researchers do mention that this is not the only possible concoction. Perhaps in the moon's low-gravity, low-pressure, low-temperature environment an easier to form and easier to handle concoction may be feasible. Additional research is needed in this particular area.

## Are rocket engines which use this propellant feasible?

Again using a paper cited by the OP the answer seems to be true. At the current state of technology (admittedly very early) it seems that only small, low power engines are possible. However, the moon is a low-gravity world with no appreciable atmosphere. Such a low-power application may be be enough for moon-station to moon-station hopping.

## Are there more suitable materials for this purpose to be found on the moon?

Almost certainly. The electrolysis of water (used to acquire the oxygen) will produce hydrogen as a byproduct, as a trivial example. Water is limited in it's locations on the Moon, however, but can be found in some locations.

# Conclusion

Based on the above, I conclude that given an established mining and manufacturing presence on the moon, and giving enough power, an aluminum-oxygen rocket engine might use locally-sourced fuel for jaunting around the moon. However, I do not expect that this technology could be used to power ships from the moon to other destinations.

• Anorthite deposits in high concentrations in lunar highlands is well established. The Isp of aluminum/LOX is hard to get references for but is surely above 200 and can be pushed somewhat by engine design. That is plenty to get on and off the moon and around cis-lunar space. There is also nothing limiting engine size. There is good reason to doubt hydrogen is available on the moon economically. There may well not be enough water at the poles, and there is none elsewhere, nor hydrogen. Nov 23, 2014 at 20:57
• Other possible fuels are listed below the question, but each has problems - magnesium is less stable, phosphorus and sulfur exist in much lower concentrations. Aluminum is the best option and that is why others who have looked at this have concentrated on it. The question listed 3 possible formats for the aluminum and did not favor the first. Once a lunar base was established (as opposed to a colony), an extraction station would be the first industrial activity on the moon. Nothing else makes sense unless you have a local fuel source. Nov 23, 2014 at 21:07
• I believe that we agree on most points other than the priority that would be given to local production. As that is speculation on both our parts, I see no reason to stress my viewpoint. Depending on the goals of the colony, either option could work. Thanks. Nov 24, 2014 at 6:35
• Then your answer needs heavy editing. People often read answers but not the comments below. If we agree, many points in your answer need to be changed. The point about local fuel production can be made by noting that a rocket equivalent to a Saturn V launched to the moon to deliver fuel could only supply a base with about 5 tons of it. Generously allowing for cost savings due to potentially reusable lower rocket stages, let's say that can be done for \$100 million. That is still \$20 000/kg, and it won't go far. Spend \\$5 billion on Al/LOX production, you recoup it with 250 tons of product. Nov 24, 2014 at 15:22
• The whole thing is speculation. I speculated in my answer, and you counter-speculated, and I agreed that your counter speculation is also valid. But they are both still speculation. You are welcome to edit your speculation into the answer if you like, but the answer can never be "complete" as it is still speculation. Nov 24, 2014 at 16:01

I'm going to go out on a limb and suggest that the technology to refine the ore works, as well as the ability to make the rocket. Here's a few things for thought:

1. Assuming an ISP of, say, 200, a proper mixture of 1/3rd mass, 2/3rd fuel would be sufficient to achieve lunar orbit. For a 4/5 fuel ratio, you could return directly to Earth.
2. Producing the LOX at the Moon would be a fairly difficult problem, as it would need to be kept cool the entire time.
3. Solids have never really been used in rockets before (Aside from solid rockets, which is a different mechanism) This would be a hybrid type rocket, pumping solids, which seems challenging. Also, you'd have to have the aluminum as a dust, which might be abrasive.
4. This seems like a fairly risky thing to do, based primarily on #3, for a manned mission. You gain a relatively small weight boom, but with a much more risky rocket.

I could see this as a return stage for lunar mining mission, but not for a manned mission. ISRU just isn't as important for a Lunar mission as it would be for a Mars mission.